1. Field of the Invention
The present invention relates to a method and apparatus for biogas purification.
2. Description of the Related Art
Biological transformation of organic material in landfills produces biogas as a result of anaerobic digestion. The organic material includes energy crops, food and farm waste, such as animal manure. Biogas is a mixture of 55-65% methane (CH4), 35-45% carbon dioxide (CO2), and 2-5% nitrogen (N2), with trace amounts of hydrogen sulfide (H2S), carbon monoxide (CO) and water vapor (H2O). CH4 is the main constituent of biogas and is twenty times more potent as a greenhouse gas than CO2. Under normal conditions biogas gas from landfills escapes into the atmosphere and contributes to global warming through the addition of greenhouse gases, such as CO2 and CH4. Accordingly, agricultural methane offsets, e.g. carbon credits, are being offered to farmers to minimize CH4 release to the atmosphere, and alternative uses for biogas have drawn increased interest as a renewable energy source due to its CH4 content.
Anaerobic digestion is a process where microorganisms decompose organic material and produce biogas as a byproduct. Anaerobic digestion generally occurs in two steps: conversion of organic wastes to organic acids such as acetic acid and propionic acid; and transformation of organic acids to biogas. Solids remaining after biogas production are referred to as digestate, which is high in phosphorous and fiber, and has a liquid fraction high in nitrogen, which can be used for fertilization and soil management.
Processing animal manure by anaerobic digestion has received attention as a method to produce biogas, as well as to reduce odors and manage nutrients flowing into the surrounding environment. Conventional biogas processing systems include U.S. Pat. No. 8,007,567 to Roe, et al., and U.S. Publ. No. 2010/0107872 A1, U.S. Publ. No. 2010/0021979 A1 and U.S. Publ. No. 2011/0023497 A1 to Bethell, Facey et al. and Assmann, respectively, the contents of which are incorporated herein by reference.
Anaerobic digesters typically utilize a series of processes in which microorganisms break down biodegradable material in the absence of oxygen. Three factors are considered to optimize biogas yield in the anaerobic digester: pH control; temperature, generally maintained at 100° F.; and an amount of water added to the digestate.
The net energy produced in the anaerobic digester is about 18,000 BTU/day per dairy cow (385 kW-hour/day equivalent per dairy cow), assuming that 35% of the energy is used for digester heating. The capital cost of a digester varies from $200-$700 per 1000 lbs. live weight of digestate being processed, with an estimated annual operating cost between $11,000-$51,000 per 1000 lbs. live weight. In the U.S., several types of digesters are in use, including a plug-flow type, complete mix type, and covered lagoon type.
An anaerobic digester will produce approximately 20-25 megajoules per cubic meter (MJ/m3), with a typical biogas composition shown in Table 1.
TABLE 1ComponentContent, vol. %CH455-70 vol. %CO230-45 vol. %H2S200-4000 ppmAmmonia (NH3)0-350 ppmHumiditySaturated
CH4 is the primary desirable constituent as raw biogas exits a digester. Other gases, including CO2, H2S, NH3, N2, CO, and hydrogen (H2) will lower a biogas heating value, and are preferably removed. In addition, CO2 and H2S cause corrosion, and are preferably separated from biogas, generally by absorption in aqueous solutions by monoethanolamine and diethanolamine, as summarized in Table 2.
TABLE 2ComponentImpactRemoval MethodsCO2Heating value reductionWater or caustic scrubbing;Solid or liquid absorption;Pressure separationH2SHighly corrosivePassing through iron sponge or wood shavings mixed with iron oxide below 500 ppmH2OCondensation; metal Passing through frost-proof surface corrosion when condenserscombined with H2S
Stepwise removal of CO2, H2S, and H2O enriches biogas to a purified CH4 composition and raises the biogas heating value. However, current purification methods of biogas substantially add to CH4 production cost and hinder use of biogas as a renewable energy source.
For example, conventional methods that use polymeric membranes of high permeability and selectivity for CO2/H2S are limited in that they are energy-intensive, cause pollution, require solvent-regeneration, have large space requirements, have high labor costs for control and maintenance, and the corrosive nature of the solvents causes membrane fouling. Purification by liquefaction is expensive because liquefaction requires extreme conditions, i.e., biogas liquefies at −82.5° C. under 4.75 MPa.
Water scrubbing technology is simple and remains the preferred method of purifying biogas. For example, purification of raw biogas having an initial CH4 content of 55%-65% in a digester based on the water scrubbing technology will typically result in a final methane content of 75%-95% (equivalent to 28.7 MJ/m3 gas energy output) in a 210 m3 gas/day production plant that utilized water under pressure at 1.3 MPa. A corresponding energy requirement of a plant is 1082 MJ/day, yielding an overall plant efficiency of 70%. However, two major limitations of biogas purification by water scrubbing are the need for a large amount of water, with such water requirement being prohibitive in areas having scarce water resources, and utilization of water spray under a high pressure of about 1.3 Mpa, typically requiring a steel tower for such spray.
An alternative method of biogas purification is to form clathrates, i.e. hydrates, of biogas impurities and to separate CH4 from the biogas impurities to yield pipeline quality CH4. Pipeline natural gas is generally defined as being composed of at least 70% methane by volume or having a gross calorific value between 950 and 1100 BTU per standard cubic foot.
FIG. 1 is a graph illustrating temperature and pressure conditions for clathrate formation of H2S, ethane (C2H6), CO2, CH4, propane (C3H8), and isobutane (C4H10). Clathrates are ice-like cages, which form from biogas by selective manipulation of temperature and pressure conditions. Water in the liquid phase facilitates clathrate formation when certain small gases are present, such as CH4, CO2, H2S, C2H6, C3H8 and C4H10. The temperature and pressure conditions for clathrate formation are dependent on a particular gas molecule and can form from other larger hydrocarbon molecules and oxygenated hydrocarbon molecules such as tetrahydrofuran. Deposits of naturally occurring methane hydrates demonstrate a clathrate phenomenon under temperature and pressure conditions of T=2-4° C. and P˜10 MPa, with CO2 clathrate formation occurring under temperature and pressure conditions of T=2-4° C. and P<2 MPa, while H2S clathrates form at T=4° C. and P=2 MPa.
The conventional methods of clathrate formation from biogas described above employ stationary systems, which are impractical for biogas purification since the amount of biogas that is output from landfills, bio digesters, and wastewater treatment plants fluctuates and eventually declines in total amount of biogas output.
Accordingly, there is a need for an economical, mobile, safe and environmentally friendly alternative technology for purifying biogas, particularly to overcome the need of conventional water-scrubbing technologies for constant fresh water feed, which competes with more important daily usage of water for drinking and other basic human requirements.